Blade granulator and method for the production of cut bodies

ABSTRACT

A granulating mill for the production of cut pieces from viscous materials and a process for producing such cut pieces are described. The mill comprises gaps between the knives ( 1, 2 ) which are at least partially covered with a spray guard ( 4 ) which is fixed to the rotor ( 5 ).

The invention relates to a granulating mill for the production of cutpieces from viscous materials and a process for producing cut pieces.

A plurality of methods are used for the production of granules or cutpieces from materials that are in the viscous state.

One method consists in feeding the material to a roll mill. The rollsare maintained at a suitable temperature. On the rolls is situated abead, which is pressed vigorously.

The freshly supplied material is passed onto the bead and worked intothe bead. At the same time the material cools down to such an extentthat a tough material of suitable viscosity is obtained.

There is drawn off from the rolls a ribbon, e.g. of thermoplasticpolymer, which is cooled. Said ribbon is then chopped with a granulator,so that “cubed granules” are obtained. Other shapes may also be cut orstamped out of the ribbon. Such a procedure is e.g. conventional in theprocessing of doughs in bakeries for the production of bakery products.

A further method consists in pressing out directly via a nozzle a ribbonwhich is cooled and cut or punched into shapes as above. Thispresupposes that the liquid may be pressed out of a nozzle in a tough,highly viscous state.

A further method consists in pressing out via a nozzle with manyopenings. Threads are pressed out, the diameter of which may in additionbe varied by stretching. After suitable cooling, threads may either bespun or they may be chopped and granules of a particular shape, mostlyin cylinder form, be obtained.

A further method consists in pressing out via a nozzle with manyopenings. The exiting “threads” are cut directly at the nozzle by meansof a cutting device, in most cases knives. Depending on the exit speedand the cutting frequency, cylindrically shaped grains of particularlength are obtained.

The invention is based on the last-mentioned process, which has becomemore and more popular in recent times.

Critical is the viscosity of the material leaving the nozzle, in orderthat cutting is in fact possible. Cutting is impossible withlow-viscosity material. With increasing viscosity, the cut pieces startto become sticky and oily. This leads to glazing, caking and the bindingof cut pieces to one another.

Only when the viscosity is sufficiently high are sufficiently solid cutpieces obtained which are able to exist independently.

The cut pieces so obtained are in the case of circular nozzle openingsconventionally often far removed from the anticipated cylindrical shape.

Depending on the material, spherical, lens-shaped or shell-shapedgranules, for example, are obtained (e.g. in the case of thermoplasticpolymers).

The causes of this may be: the surface tension of the material, thevarying cooling rate of the inner and the outer material, whereinstresses in the particle are frozen in, as well as elastic relaxationafter the exit from the nozzle.

A further reason for the deformation of the cut pieces are mechanicalforces during the cutting, during the removal from the cutting knife orduring impact against walls or fittings. Longish, flattened cut piecesare often the result of mechanical deformation.

Deformation of the cut pieces is undesirable in most cases.

Many materials, particularly plastics in the viscous state, have poorthermal conductivity and their volume is temperature-dependent to alarge extent.

In the case of many materials the surface cools rapidly and hardens.Because of the poor heat conductivity the inside of a particle remainson high temperature, low viscosity and large volume for a prolongedperiod.

Given the slow rate of cooling of the inner part of a granule, saidmaterial undergoes a diminution in volume.

Since the outer envelope is already dimensionally stable, there forms onthe inside a cavity in which vacuum is present, the so-called vacuole.With very slow cooling said vacuole does not form. As soon as saidcavity is opened, e.g. by mechanical action or due to excessively thinwalls, surrounding air or surrounding liquid may penetrate into saidcavity. During the further processing, quality losses may occur due tothe materials contained in the opened cavities. Enclosed water, forexample, causes a foaming during the further processing.

The rate of cooling of the cut piece after the cutting therefore has agreat influence on its shape and on its content of vacuoles.

The most common methods of cooling are:

-   -   placing of the cut piece in water,    -   spraying with water,    -   treatment with vapour, air or other coolants.

It is also a question, immediately after the cutting, of stabilising theshape of the cut piece by suitable cooling and of preventing adhesion tothe cutting tools. The cooling must therefore commence already duringthe cutting. This may be achieved e.g. by spraying of the knives. Theknives in their turn swirl the coolant and ensure a contact with coolantmist also during the disengagement of the cut piece from the knife.

A known method of cooling the rotating knives is axial spraying withcoolant.

It is disadvantageous that the nozzle openings are then also sprayed andcooled at the same time.

The nozzle openings cool down and close many opening wholly orpartially. The result is that the nozzle holes that have remained openhave a greater throughput, i.e. form greater cut pieces, and thosepartially opened form very small cut pieces (glitter, platelets). Abroad particle size spectrum is obtained.

The problem of undesirable spraying of the nozzle holes is also to besolved by the invention.

The object of the invention is to provide a granulating mill and aprocess for producing cut pieces which avoids the disadvantages of theknown apparatuses and supplies a comparatively better and more uniformproduct quality in particular of polymer granules.

The object is achieved according to the invention by the fact that onthe granulating mill the gaps between the knives are covered at least inpart with a spray guard which is fixed to the rotor.

The invention provides a granulating mill for the production of cutpieces from viscous materials, in particular from polymers, with aliquid spray device, comprising at least one rotor with a plurality ofknives arranged on the periphery of the rotor and an extrusion devicewith die plate which during the operation of the rotor is swept by theknives, characterised in that the gaps between the knives are covered atleast in part with a spray guard which is fixed to the rotor.

For example, during the spraying of the knives with coolant the gapsbetween the knives are protected against coolant by fitting a sprayguard as a continuation of the knives. Said spray guard rotates togetherwith the knife. It is attached preferably directly behind the knife andextends to directly before the following knife. The cutting face of thefollowing knife must not be covered, viewed axially, in order that themouldings cut there are able to snap off. The spray guard is set at asufficient distance from the nozzle holes of the die plate. The cutpieces slide along on the spray guard, wherein they come into contactwith coolant located on the spray guard and stabilise their shape.Grooves, slits or plates optionally imprinted or placed on the sprayguard additionally control in a special embodiment the path of the cutpiece and its residence time on the spray guard.

Preferably the spray device consists of one or more mist nozzles forgenerating a liquid mist, which are aligned in the direction of the dieplate. The spray guard prevents a direct spraying of the nozzle openingswith the spray liquid.

In a preferred embodiment of the invention the spray device is arrangeddirected onto the rear side of the knives.

In a preferred embodiment of the invention the spray device is arrangedon the rotor of the knives.

Special advantages are achieved with an embodiment of the granulatingmill in which the liquid supply for the spray device is arranged on orin the rotor and rotates with it, and is passed through pipes into thearea of the knives, wherein the outlet opening of the liquid supply isarranged in the area of the knives and points in the direction of thedie plate.

A further improvement may be achieved in a particularly preferredembodiment of the invention by coolant (e.g. water) being sprayed ontothe head of the rotor. A “catching device” is located there. From thelatter, channels and pipes lead to the knives. According to theprinciple of the rotary pump, the coolant runs along the knife arms tothe knives in an outward direction and is there distributed on thecutting face. It is beneficial to allow the outlet opening of thecoolant to end at some distance before the knives, in order that thecoolant sprays onto the knife axially.

The quality of the cut material may be further improved if in apreferred embodiment the knives are fixed to the rotor by springs andare constructed so as to be conveyable across the die plate by springs.A smooth cut is ensured in this way.

Depending on the application, the nozzles of the die plate areconstructed in special shapes, e.g. star shape or else Father Christmasshapes (application to bakery products).

The invention also provides for a process for producing cut pieces orgranules from viscous materials by means of a granulating mill withspray device, characterised in that the die plate, in the case of anaxial spraying of the knives with a liquid, is protected against directspraying with the liquid by a spray guard attached to the knife clip androtating with it.

Preferably a granulating mill according to the invention and describedabove is used.

There may be used as viscous material a polymer melt, in particular athermoplastic polymer, selected from the group: polyamide,polycarbonate, polyvinyl chloride, polyolefin, in particularpolypropylene, ABS, SAN resin or polyurethane or mixtures of theabove-mentioned polymers.

There is used as viscous material particularly preferably polyamide, inparticular preferably polyamide which is in particular reinforced withglass fibres and contains a proportion of glass fibres of 15 to 50 wt %,particularly preferably of 20 to 30 wt % of glass fibres.

A cooling liquid, in particular water, may be used as spray liquid.

There serves alternatively as spray liquid a heating fluid, inparticular oil, which leads to the heating of the granules. Saidprocedure is used with advantage in the production of bakery products.

The invention will be explained in detail below with reference to thefigures by the examples, which however do not represent a limitation ofthe invention.

FIG. 1 shows a partial view of the granulating mill according to theinvention with opened casing.

FIG. 2 shows an alternative supplying of the cooling liquid.

EXAMPLE

Two knives 1, 2, which are arranged on the periphery of a rotor 5,rotate axially (cf. FIG. 1; only the view onto the die plate of thegranulator without casing and periphery is shown here). The speed comesto 1200 r.p.m. The knives 1, 2 are 10 cm wide on the cutting face. Thespray guard 4 is constructed 1 cm wider than the knives 1, 2 in both aninward and outward direction. At the end, namely before the respectivefollowing knife 1 or 2, the spray guard 4 is raised by 2 cm and is thusspaced about 2 cm from the nozzle holes 6. It ends 0.5 cm before thecutting face of the following knife.

The spray guard 4 is a semicircular-shaped ring of stainless steel witha central angle of 170°, an outer diameter of 31 cm and an innerdiameter of 19 cm.

The die plate 3 with the nozzles 6 has an outer diameter of 30 cm and aninner diameter of 20 cm. The nozzle holes 6 have a diameter of 0.35 cm.

Water with a temperature of 80° C. was sprayed axially onto the knives1, 2 and the spray guard 4 as coolant.

In a modified granulator device (cf. FIG. 2) the coolant (water) is fedto the additional pipes 7 through the rotor 5 and sprayed via theterminal openings 8 in the area of the knives 1, 2.

1. Granulating mill for the production of cut pieces from viscousmaterials, in particular from polymers, with a liquid spray device,comprising at least one rotor (5) with a plurality of cutting knives (1,2) arranged on the periphery of the rotor (5) and an extrusion devicewith die plate (3) which during the operation of the rotor (5) is sweptby the knives (1, 2), characterised in that the gaps between the knives(1, 2) are covered at least partially with a spray guard (4) which isfixed to the rotor (5).
 2. Granulating mill according to claim 1,characterised in that the spray device consists of one or more mistnozzles for producing a liquid mist which are aligned in the directionof the die plate (3).
 3. Granulating mill according to claim 1characterised in that the spray device is arranged directed towards therear side of the knives (1, 2).
 4. Granulating mill according to claim 1characterised in that the spray device is arranged on the rotor (5) ofthe knives.
 5. Granulating mill according to claim 4, characterised inthat the liquid supply of the spray device is arranged on or in therotor (5) and rotates with it, and is fed through pipes (7) into thearea of the knives (1, 2), wherein the outlet opening (8) of the liquidsupply is arranged in the area of the knives (1, 2) and points in thedirection of the die plate (3).
 6. Granulating mill according to claim 1characterised in that the knives (1, 2) are fixed to the rotor (5) bysprings and are conveyable across the die plate (3) by springs. 7.Process for producing cut pieces or granules from viscous materials bymeans of a granulating mill with spray device, characterised in thatduring an axial spraying of the knives (1, 2) with a liquid, the dieplate (3) is protected against direct spraying with the liquid by aspray guard (4) attached to the knife clip and rotating with it. 8.(canceled)
 9. Process according to claim 7, characterised in that thereis used as viscous material a polymer melt, in particular athermoplastic polymer, selected from the group consisting of polyamide,polycarbonate, polyvinyl chloride, polyolefin, ABS, SAN resinpolyurethane and mixtures of the above polymers.
 10. Process accordingto claim 9, characterised in that there is used as viscous materialpolyamide, preferably polyamide which is reinforced in particular withglass fibres and contains a portion of glass fibres of 15 to 50 wt %,particularly preferably of 20 to 30 wt % glass fibres.
 11. (canceled)12. (canceled)
 13. The process according to claim 7, comprising cuttingthe viscous material in a granulating mill with a liquid spray device,comprising at least one rotor (5) with a plurality of cutting knives (1,2) arranged on the periphery of the rotor (5) and an extrusion devicewith die plate (3) which during the operation of the rotor (5) is sweptby the knives (1, 2), characterised in that the gaps between the knives(1, 2) are covered at least partially with a spray guard (4) which isfixed to the rotor (5).